In Carcosa, you summon forth the home of the Tattered King, one tile at a time, using your cultists to control leylines, conduct rituals, and influence the districts of Carcosa itself. If they displease you, you can sacrifice them to the dark waters of Lake Hali for the pleasure of the King.
Each player leads a Lovecraftian cult, taking turns using their prophet to select a tile representing a section of Carcosa from the cult board and adding it to the growing magnificent landscape of ritual sites, leylines, towers and domes. Players can then deploy cultists to control a leyline, conduct a ritual, or influence the districts they have just summoned.
But Carcosa is no normal city! You must stabilize your summoned tiles with power drawn from leylines or they may be reshaped by other cults!